1. Field of the Invention
The present invention relates to a touch sensor which detects contact of a finger, etc. with a plate.
2. Description of the Related Art
As to touch switches electrodes are installed on a glass surface with EL as back lighting, various systems are designed.
The first system is a method to detect the hum induced by the commercial power supply to a human body. It falls into two categories to simply amplify the voltage and use it as a signal source and the other is to identify frequency and use it as a signal source.
The second system is a method to use leak current of the commercial power supply.
The third system is a method to use a PLL oscillator circuit and detect a sudden change in frequency when a finger touches the electrode.
The fourth system is a method to connect two capacitors in series, apply a.c. signals from one side and detect the signals from the opposite side, install an electrode at a midpoint of two capacitors. The attenuation of signal is detected with a comparator by the impedance formation between the electrode and the ground when the electrode is touched with a finger. This system has been commercialized, for example, in IC MN1976 of Matsushita Electric Industrial Co., Ltd.
The fifth system is a method to detect current flowing between the two electrodes of a high resistor when the electrodes are connected by a touch of a finger. This system has already been applied in IC (SO56H) of Siemens company and IC (ML238/9) of Plessey company.
However, the five systems described above have several problems.
In the first system the high level of induced hum from EL could cause malfunction of the system and input impedance of the amplifier must be lowered to about 1 k.OMEGA.. The effect of the induced hum from EL is also susceptible to which of the time electrodes of EL is connected to the grounded electrode of the commercial power supply. This system can be adopted when EL is operated at a frequency higher than the commercial power frequency, but in the location in which there is not more than one power line in the vicinity of a human body, the input level may become insufficient.
The second system has restrictions in polarity of the commercial power supply and contains a danger of electric shock, and it cannot be said to suit the general purpose. In ICs of Plessey company, high resistors are inserted to one side of the double-pole electrode and the system is connected to 240 VAC, but this is not intended for popular application.
The third system shows small amount of frequency change due to high resistance of several thousands of K.OMEGA. of the electrode of the EL surface. The lock range must be narrowed, bringing up problems of stability and costs.
The fourth system can operate properly when the electrode is made with a metal plate with a lower resistivity. However in the case of a transparent electrode film installed on the glass surface the voltage drop caused by a finger touch is very small. A stable signal source greater than 30 V and a stable reference voltage are required, resulting in problems of costs and size.
The fifth system is the simplest of all, but the EL with the electrode installed on the glass surface is seriously affected with a problem of condensation of water vapor, and therefore it lacks reliability as a touch switch.